This invention relates to a hardness testing machine of the kind (hereinafter called the kind specified) comprising a support for an object which is to be tested, an indenter which, in use, forms an indentation in the object, and a mechanism for urging the indenter against the object under test.
In known hardness testing machines of the kind specified, the indenter is secured to the lower end of a vertical rod, generally known as the indenter spindle, and the mechanism for urging the indenter against the object under test includes at least one lever connected with the rod at the upper end of the latter. In use, the indenter spindle transmits downwardly directed forces from the lever to the indenter.
In use of a first known type of hardness testing machine of the kind specified, the area of the indentation is determined by measuring a dimension of e.g. the diameter of, the indentation, generally by means of a travelling microscope incorporated in the testing machine.
With a second known type of testing machine of the kind specified, the increase in depth of the indentation which is caused by a predetermined increase in the force which urges the indenter against the object under test is determined. In this second known type of machine, there is connected to the upper end of the indenter spindle a mechanism which transmits movement of the spindle to an indicator and also amplifies this movement so that movement of the indicator which corresponds to a very small movement of the indenter can readily be observed. One source of error in measurements of hardness made with this second known type of machine is vertical movement of the indenter relative to the indenter spindle when under load.
A further source of error, which is more important in connection with the second known type of machine referred to above, but may also be significant in the first known type of machine referred, is the friction which impedes movement of the indenter spindle and friction in the mechanism for urging the indenter against the object under test.
A further disadvantage of known hardness testing machines of the kind specified is that such machines cannot be used, with the indenter mounted directly on the indenter spindle, to test the hardness of internal surfaces of workpieces. To enable the known machines to be used for such measurements, the machines are adapted by the interposition of a goose neck between the indenter spindle and the indenter. Generally, a range of different goose necks is provided with known machines of the kind specified to enable hardness measurements to be made on various surfaces of workpieces of various sizes and shapes. The use of goose necks in machines of the kind specified has a number of disadvantages. The assymetrical shape of a goose neck results in a greater deflection of the machine, when under load, than occurs when the indenter is mounted directly on the spindle. When a goose neck is used, it is difficult to maintain proper alignment of the indenter with the indenter spindle. Large goose necks, which are necessarily heavy, require indenter spindles of special form to enable such goose necks to be properly secured to the spindle. Particularly in the case of heavy goose necks, the mechanism for urging the indenter against the object under test must be adjusted to compensate for the weight of the goose neck which is applied to the indenter. When such adjustments have been made, and the machine is to be used without a goose neck, special weights can be added to the mechanism to compensate for the absence of the goose neck, but this complicates use of the machine. It will also be appreciated that the provision of a variety of goose necks with each machine contributes significantly to the overall cost.
A further disadvantage of the second known type of machine hereinbefore referred to is that an erroneous indication of the increase in depth of the indentation is given in any case where non-elastic collapse of the object being tested occurs at that face of the object which engages the support, i.e. the face remote from the indenter. Such non-elastic collapse is likely to occur in cases where the surface of the object under test which is engaged with the support is not a machined surface, for example a surface formed by casting or forging.
A still further disadvantage of known machines of the kind specified, both when used with a goose neck and when the indenter is attached directly to the spindle, is that visibility of the surface area of the object at which the hardness is to be tested is restricted.